Interim Guidance on Case Definitions to be Used For Investigations of Swine-Origin Influenza A (H1N1) Cases*

April 30, 2009, 10:00 AM ET

This document provides interim guidance for state and local health departments conducting investigations of human cases of swine-origin influenza A (H1N1) virus (S-OIV). The following case definitions are for the purpose of investigations of suspected, probable, and confirmed cases of S-OIV infection.

Acute febrile respiratory illness is defined as a measured temperature 100 degrees Fahrenheit and recent onset of at least one of the following: rhinorrhea or nasal congestion, sore throat, or cough.

Case Definitions for Infection with Swine-origin Influenza A (H1N1) Virus (S-OIV)

A confirmed case of S-OIV infection is defined as a person with an acute febrile respiratory illness with laboratory confirmed S-OIV infection at CDC by one or more of the following tests:

1. real-time RT-PCR
2. viral culture

A probable case of S-OIV infection is defined as a person with an acute febrile respiratory illness who is positive for influenza A, but negative for H1 and H3 by influenza RT-PCR

A suspected case of S-OIV infection is defined as a person with acute febrile respiratory illness with onset

• within 7 days of close contact with a person who is a confirmed case of S-OIV infection, or
• within 7 days of travel to community either within the United States or internationally where there are one or more confirmed cases of S-OIV infection, or
• resides in a community where there are one or more confirmed cases of S-OIV infection.